Vertical rate reference ramps may be generated by coupling a current source to a capacitor during the vertical trace time. The current source charges the capacitor developing a ramp or sawtooth voltage. The vertical sync pulse defines the start of vertical retrace and is used to discharge the voltage developed across the capacitor. At the end of the vertical sync pulse, the current source resumes charging the capacitor to produce a repetitive sawtooth waveform synchronized to the vertical sync pulse.
Vertical rate reference ramps may also be generated by coupling a voltage source to a capacitor during the vertical retrace interval. The voltage source charges the capacitor to the maximum voltage of the reference ramp, and then during vertical trace, the voltage source is decoupled from the capacitor and the capacitor is allowed to discharge through a resistor. The voltage across the capacitor decreases, thereby defining a ramp. This process is repeated at the vertical scanning rate, in synchronism with the vertical sync pulse.
The repetitive ramp or sawtooth reference signal is coupled to the vertical output amplifier which drives the vertical deflection winding. A sawtooth current is generated in the vertical deflection winding and the resultant magnetic field causes deflection of the electron beam within the cathode ray tube.
A reference ramp which is produced by discharging a capacitor through a resistor is not linear. The voltage across the capacitor decays exponentially according to the associated RC discharge time constant, i.e., the slope of the voltage waveform is progressively shallower over time. Although various deflection circuit parameters can be adjusted to reduce the effects of a nonlinear reference ramp, the exponential flattening in the reference ramp is undesirable since it will produce nonlinearity of displayed picture in the vertical direction.
The reference ramp signal from the ramp generator is coupled to a vertical output amplifier through a differential amplifier. A DC blocking capacitor may be coupled in series with the deflection winding and the output of the vertical output stage for producing an AC deflection current in the winding. The vertical output amplifier is essentially a voltage-to-current converter. The reference ramp signal is coupled to one input of the differential amplifier. Feedback paths couple the output of the output amplifier to both inputs of the differential amplifier for setting the gain of the output driver stage and for shaping the output signal. A negative DC feedback path determines the DC level of the output, and an AC negative feedback a signal developed from the voltage across a current sampling resistor coupled in series with the deflection coil provides control of the output level.
The output voltage of the output amplifier has components of a ramp voltage and a parabolic voltage. The ramp component is generated by the deflection current across the resistance of the winding and the resistance of the series connected current sampling resistor. The parabolic component is generated by integration of the deflection current by the DC blocking capacitor.
To provide S-shaping of the deflection current, the output voltage is integrated which results in the parabolic component being double integrated forming a third-order correction waveform. The integrated output voltage is then supplied to the non-inverting input of the differential amplifier for driving the output amplifier so as to generate an S-shaped deflection current.